Awnser please dont have much time

Evaluate the expression for the given value of the variable 7a - 4a for a =8

In the polynomial below, what number should replace the question mark to produce a difference of squares? x2 + ? x - 36

Assume that there is a 11% rate of disk drive failure in a year. a. if all your computer data is stored on a hard disk drive with a copy stored on a second hard disk drive, what is the probability that during a year, you can avoid catastrophe with at least one working drive? b. if copies of all your computer data are stored on four independent hard disk drives, what is the probability that during a year, you can avoid catastrophe with at least one working drive?